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Register a new userAtomic spin sensitive dissipation on magnetic surfaces
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We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.
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The possibility of having positronium (Ps) physisorbed at a material surface is of great fundamental interest, since it can lead to new insight regarding quantum sticking and is a necessary first step to try to obtain a Ps$_2$ molecule on a material host. Some experiments in the past have produced evidence for physisorbed Ps on a quartz surface, but firm theoretical support for such a conclusion was lacking. We present a first-principles density-functional calculation of the key parameters determining the interaction potential between Ps and an $alpha$-quartz surface. We show that there is indeed a bound state with an energy of 0.14 eV, a value which agrees very well with the experimental estimate of $sim0.15$ eV. Further, a brief energy analysis invoking the Langmuir-Hinshelwood mechanism for the reaction of physisorbed atoms shows that the formation and desorption of a Ps$_2$ molecule in that picture is consistent with the above results.
We present a thorough theoretical assessment of the stability of non-collinear spin arrangements in small palladium clusters. We generally find that ferromagnetic order is always preferred, but that antiferromagnetic and non-collinear configurations of different sorts exist and compete for the first excited isomers. We also show that the relative stability of all these states is rather insensitive to the choice of atomic configuration for the pseudopotential used and to the approximation taken for the exchange and correlation potential. This result stands in stark contrast with the situation found for the bulk phases of palladium.
We attempt to simulate the heterogeneous nucleation of ice at model silver-iodide surfaces and find relatively facile ice nucleation and growth at the Ag+ termi nated basal face, but never see nucleation at the I- terminated basal face or the prism and normal faces. Water molecules strongly adsorb onto the Ag+ terminate d face to give a well-ordered hexagonal ice-like bilayer that then acts as a template for further ice growth.
Alternating layers of granular Iron (Fe) and Titanium dioxide (TiO$_{2-delta}$) were deposited on (100) Lanthanum aluminate (LaAlO$_3$) substrates in low oxygen chamber pressure using a controlled pulsed laser ablation deposition technique. The total thickness of the film was about 200 nm. The films show ferromagnetic behavior for temperatures ranging from 4 to $400 ^oK$. The layered film structure was characterized as p-type magnetic semiconductor at $300 ^oK$ with a carrier density of the order of $10^{20} /cm^3$. The undoped pure TiO$_{2-delta}$ film was characterized as an n-type magnetic semiconductor. The hole carriers were excited at the interface between the granular Fe and TiO$_{2-delta}$ layers similar to holes excited in the metal/n-type semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS) devices. The holes at the interface were polarized in an applied magnetic field raising the possibility that these granular MOS structures can be utilized for practical spintronic device applications.
We present calculations of the magnetic ground states of Cr trimers in different geometries on top of a Au(111) surface. By using a least square fit method based on a fully relativistic embedded-cluster Greens function method first we determined the parameters of a classical vector-spin model consisting of second and fourth order interactions. The newly developed method requires no symmetry constraints, therefore, it is throughout applicable for small nanoparticles of arbitrary geometry. The magnetic ground states were then found by solving the Landau-Lifshitz-Gilbert equations. In all considered cases the configurational energy of the Cr trimers is dominated by large antiferromagnetic nearest neighbor interactions, whilst biquadratic spin-interactions have the second largest contributions to the energy. We find that an equilateral Cr trimer exhibits a frustrated 120$^circ$ Neel type of ground state with a small out-of-plane component of the magnetization and we show that the Dzyaloshinsky-Moriya interactions determine the chirality of the magnetic ground state. In cases of a linear chain and an isosceles trimer collinear antiferromagnetic ground states are obtained with a magnetization lying parallel to the surface.
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